1. Field of the Invention
The invention relates generally to pulsed high voltage power supplies and, more particularly, to a pulsed high voltage power supply for use within a radiography system.
2. Description of Related Technology
Generally speaking, radiography and fluroscopy systems include a radiation source that emits high energy photons (e.g., X-rays, gamma rays, etc.) toward a target object and a radiation detector that measures the energy level of photons which have passed through the target object. The radiation detector may, for example, be a charge coupled device (CCD) or a fluoroscope that detects the differential transmission of the high energy photons through the target object to produce images of structures within the target object. These internal images of the target object may be developed and displayed using photographic film and/or may be displayed using a video monitor.
Radiography systems are used in a wide variety of applications and are particularly useful in examining and diagnosing problems with the internal structures of a target object. For instance, in the field of medical diagnostics, medical practitioners use radiography systems to produce radiographic images that reveal the internal conditions of a patient's body. Specifically, radiography systems may be used to assess the condition of damaged or diseased organs, bones, etc. and/or may be used to determine the location of a foreign object within the patient's body. Additionally, radiography systems may be used to determine the internal conditions of machinery and components of a physical plant (e.g., pipes, valves, etc.) to perform preventative maintenance or may be used to perform quality control checks of products being manufactured within a high speed production process.
Of particular concern in using radiography systems for medical applications is that human tissues may be easily damaged by the large doses of radiation which are imparted by conventional radiography systems. Tissue damage is especially critical within the field of pediatrics because children are highly susceptible to tissue damage from exposure to high doses of radiation. In fact, medical guidelines recommend X-ray exposure levels for children that are substantially reduced with respect to the levels acceptable for adult patients. As a result, important developments within the field of radiography have been directed to minimizing the exposure of patients (and medical personnel operating the radiography equipment) to radiation while maintaining or improving radiographic imaging capability.
Additional advances in radiography have been directed to the development of quasi real time imaging capability. With quasi real time imaging, successive radiographic images are acquired at a rate that is perceptible to the human eye (e.g., less than 30 updates or frames per second) and then displayed via a video monitor to a user. Quasi real time radiographic images are particularly useful within the field of medical diagnostics because quasi real time images allow medical practitioners to inspect moving organs, such as the heart, in operation. Additionally, quasi real time radiographic images may be used to view the internal structures of subjects (e.g., patients or any other target objects) that are moving, either deliberately or inadvertently, without blurring of the images. However, because quasi real time video images are updated at rate which is readily perceived by the human eye, the video images "flicker" and, as a result, are generally difficult to view and may be of limited use for diagnostic purposes.
Still other efforts within the field of radiography have been directed to developing portable radiography systems that provide quasi real time imaging capability while addressing the above-noted need to minimize the radiation dosage imparted to a target object. Additionally, these portable radiography systems attempt to provide attributes desirable of equipment designed for field use such as a low cost, lightweight, extended battery powered operation, etc.
Conventional radiography systems typically reduce the radiation dosage imparted to the target object by pulsing the output of the radiation source. In general, these conventional pulsed radiography systems turn the radiation source on and off at a predetermined frequency and duty cycle for a predetermined period of time, which results in an integrated radiation dosage that is at or below desired safe levels. The radiographic images produced by these pulsed systems are acquired during the time intervals when the radiation source is on and are displayed to the user while the radiation source is off and until another image is acquired and ready for display. Typically, these quasi real time medical radiography systems display the images acquired while the radiation source is on using a video monitor that is synchronized with the acquisition of the images.
Traditionally, pulsed radiography systems use an X-ray tube as a radiation source. One common technique of providing a pulsed source of X-rays uses a grid controlled X-ray tube having a constant cathode to anode potential. In a grid controlled configuration, the output of the X-ray tube is gated on and off by applying a series of pulses to the grid terminal, which controls the current flowing between the anode and cathode of the X-ray tube, to generate a corresponding series of X-ray pulses that are directed toward the target object. However, grid controlled X-ray tube configurations are undesirable for many applications because grid controlled configurations result in a radiography system that is heavy, electrically inefficient, and expensive to produce.
More specifically, grid controlled X-ray tubes are significantly more expensive than non-gridded tubes. For example, a grid controlled X-ray tube may cost approximately $10,000, whereas a non-gridded tube having comparable X-ray output characteristics may only cost approximately $200. Additionally, because grid controlled configurations require a constant high voltage supply to the anode and cathode electrodes of the X-ray tube, the radiography system power supply and the grid controlled X-ray tube continuously dissipate energy and must be capable of operating under high quiescent power levels and high temperatures. These high quiescent energy levels and high operating temperatures increase system material costs, system weight, and reduce overall system performance.
In fact, many commercially available pulsed radiography systems based on grid controlled X-ray tubes, such as those manufactured by Philips Inc., employ oil cooling apparatus and/or must be periodically turned off to prevent overheating and system failure. Further, because grid-controlled X-ray tubes operate at a relatively high temperature, the life expectancy of such tubes is greatly diminished. This reduced life expectancy significantly increases operating costs over the life of the radiography system due to the high costs associated with repeated replacement of a grid controlled X-ray tube. Thus, radiography systems based on grid controlled X-ray tube configurations are undesirable for many radiography applications, particularly for field use applications requiring low cost, reliability, battery powered operation, and ease of portability.
Another common method of providing a pulsed source of X-rays turns the supply voltage (i.e., the anode to cathode voltage) of a non-gridded X-ray tube on and off at a predetermined frequency and duty cycle. Typically, such pulsed supply configurations apply a pulse waveform to the primary winding of a step up transformer and use a conventional diode-based voltage multiplier circuit to further increase the output voltage of the transformer secondary winding to generate a high voltage pulse waveform that is applied across the anode and cathode electrodes of the non-gridded X-ray tube. While these conventional pulsed supply configurations can use relatively inexpensive non-gridded X-ray tubes, they have significant drawbacks. For instance, the diode-based voltage multiplier circuit introduces a large time constant, which results in a low slew rate and a low bandwidth which, in turn, results in the application of a relatively large radiation dosage for each radiographic image.
FIG. 1 illustrates, by way of example only, a supply voltage pulse waveform 10 having a large time constant and a low slew rate such as that which would typically be found in the above-described pulsed supply voltage configurations. Because the energy level of the X-rays emitted by a pulsed supply X-ray tube varies in proportion to the supply voltage, the penetration effectiveness of the X-ray output changes over the duration of the pulse waveform 10 and only a portion of the pulse waveform 10 provides photon energy levels that are sufficient to penetrate the target object and which are useful for imaging purposes. For example, if a supply voltage of 70 kilovolts (kV) corresponds to the minimum photon energy level sufficient for penetration of the target object and imaging of structures within the target object, then only a central portion 12 of the pulse waveform 10 is useful for imaging purposes and portions 14 and 16 surrounding the central portion 12 produce photons or "soft" X-rays that are absorbed by the target object and, thus, are not useful for imaging purposes.
Furthermore, the central portion 12 of the pulse waveform 10 may produce a poor quality image because the energy level of the penetrating photons emitted within the central portion 12 varies significantly. As is generally known, a wide variation in the energy level of penetrating photons produces a "fuzzy" or unclear image of the internal structures of the target object. Some conventional radiography systems attempt to improve the quality of such unclear images by using complex software routines that selectively parse data associated with the detection of penetrating photons to effectively narrow the central region 12 and/or use complex correction algorithms to compensate for the effects of the variable energy levels of the penetrating photons. In any case, the low slew rate associated with conventional pulsed supply radiography systems is undesirable because only a small portion of the X-rays imparted to the target object are useful for imaging purposes and, as a result, the target object must be exposed to a relatively large dosage of X-rays to produce a useful image. Additionally, due to the low slew rate, the X-ray tube must remain turned on for a relatively long period of time to produce a useful image. Because the X-ray tube remains turned on for a relatively long period of time, a relatively large amount of power is dissipated by the X-ray tube and the radiography system as a whole, which increases operating temperatures of the system, reduces the operating life of the X-ray tube, prohibits efficient battery powered operation, and may require a periodic shut down of the system to prevent overheating of the system.
Yet another method of providing a pulsed source of X-rays uses a capacitive discharge configuration that is based on a "flash" X-ray radiation source, which allows a charge to build over time and which arcs over to generate an X-ray output when a breakdown voltage is reached. While these flash X-ray systems provide high slew rates and extremely narrow X-ray pulse waveforms (e.g., 50 nanoseconds in duration), flash X-ray systems are undesirable for many radiography applications because flash X-ray systems provide a relatively uncontrolled X-ray output energy level. Specifically, the arc over point of the flash X-ray device varies significantly from pulse to pulse and varies significantly over time as the flash X-ray device ages (i.e., wears due to electrode erosion). Variations in the arc-over point result in a variation in the energy level of the penetrating photons that are generated during the discharge cycle, which results in an uncontrolled and variable radiation dose on a per pulse basis. Such variability in the radiation dose and energy level results in both poor imaging capabilities and unpredictable radiation effects on the target object, which may be a human body. Additionally, flash X-ray devices utilize relatively high peak electrode currents that cause severe erosion of the electrode surfaces, which substantially reduces the life of the flash X-ray device, and cause the output beam or spot to move over time.